#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<math.h>

int main()
{
	double a = 0, b = 0, c = 0;
	double t = 0;
	double s = 0;
	scanf("%lf %lf %lf", &a, &b, &c);
	t = (a + b + c) / 2.0;
	s = sqrt(t * (t - a) * (t - b) * (t - c));
	printf("%f", s);
	return 0;
}

//3.799671